1. Field of the Invention
The present invention relates to a fixing device used for fixing a toner image transferred onto a transfer member serving as a recording member in an image forming apparatus which forms images, for example, by an electrophotographic method or the like. More particularly, the present invention relates to an improvement on the fixing device in respect to the transfer-member separability and the service lives of a fixing roller and a pressing roller.
2. Description of Related Art
The fixing device of the above-stated kind is composed of a heat source, a fixing roller arranged to fix, with heat and pressure, a toner image transferred to a transfer member to the transfer member, and a pressing roller arranged to apply a pressure from the back side of the transfer member.
Since the fixing roller applies heat while being in contact with the toner image, the fixing roller is in most cases provided with an elastic layer which is typically a rubber layer. Since, in most cases, the transfer member is a sheet having a rugged surface, such as a paper sheet, the use of the elastic layer ensures a better adhesion of the toner to the rugged surface of the transfer member. With the fixing roller provided with an elastic layer, therefore, heat can be efficiently transmitted with elasticity to the toner image on the transfer member. Generally, to improve the releasing ability for the toner, the surface of the fixing roller is coated with a fluororesin layer or the like irrespective of the presence or absence of the elastic layer.
The pressing roller comes into pressed-contact with the fixing roller to form a predetermined nip part between them. In many cases, the pressing roller is provided with an elastic layer such as a rubber layer, as in the fixing roller, for shortening heat transmission time. The toner on the transfer member is fixed by its contact with the fixing roller. However, to prevent contact of the pressing roller with the toner in forming images on both sheet surfaces and to prevent a foreign matter or smut, such as residual toner, on the fixing roller from sticking to the pressing roller, the surface of the pressing roller is also coated with a fluororesin or the like in general.
Further, the fixing roller or both the fixing roller and the pressing roller are provided with a separation claw having a pointed fore end for preventing entanglement or clinging of the transfer member to the roller. To ensure a sufficient separating effect on the transfer member with a thickness of about 100 xcexcm, the separation claw is generally arranged to have its fore end formed into a minute curved shape of a radius of curvature of 100 xcexcm or less.
The fixing device which is provided with such a separation claw shows a good separating performance for a normal transfer member. However, in the event of a transfer member which is vulnerable to heat, or has a sharp edged sectional shape at its fore end or is extremely thin in thickness, the separating ability of the fixing device becomes insufficient and thus tends to have the transfer member stick or cling to the fixing roller or to the pressing roller.
It has been known through experience that this problem can be solved by arranging the fore end of the separation claw to be very sharp measuring less than 10 xcexcm in radius of curvature. However, if the fore end of the separation claw is simply arranged to be sharp, the sharp end of the separation claw tends to pierce the surface of the fixing or pressing roller in a case where the roller is provided with an elastic layer. In such a case, the surface of the roller would be peeled off to shorten the service life of the roller.
Further, if the separation claw is arranged to have such a sharp fore end that measures less than 5 xcexcm, it increases a dynamic load on the surface of the roller to cause large abrasion of the surface of the roller and thus also shortens the service life of the roller.
Therefore, it has been hardly possible with the conventional fixing device to make the long service life of the roller compatible with a good separability for a transfer member which tends to stick and cling to the roller.
It is a general object of the invention to provide a fixing device improved in recording-member separability and service life of a roller.
It is a more specific object of the invention to provide a fixing device, which comprises a roller arranged to come into contact with a recording member bearing an unfixed image thereon so as to fix the unfixed image to the recording member, and a separation member arranged to come into contact with the roller so as to separate the recording member from the roller, wherein a fore end of the separation member on a side for coming into contact with the roller is formed in a curved surface and intrudes into the roller, and an amount of intrusion of the separation member into a surface of the roller is not less than a radius of a circle of curvature of the curved surface of the fore end of the separation member and not more than a diameter of the circle of curvature.
It is another object of the invention to provide a fixing device, which comprises a roller arranged to come into contact with a recording member bearing an unfixed image thereon so as to fix the unfixed image to the recording member, and a separation member arranged to come into contact with the roller so as to separate the recording member from the roller, wherein a fore end of the separation member on a side for coming into contact with the roller intrudes into the roller, and the following condition is satisfied:
90xc2x0xe2x89xa6xcex8 less than xcex80
where xcex8 is an angle which a tangential line of the fore end of the separation member at a position where the fore end of the separation member intersects a surface of the roller before the separation member comes into contact with the roller makes with a tangential line of the surface of the roller at the above-mentioned position before the separation member comes into contact with the roller, and xcex80 is an angle which a surface of the separation member for guiding the recording member makes with a tangential line of the surface of the roller at a position where a line of the surface of the separation member in a direction of guiding the recording member intersects the surface of the roller before the separation member comes into contact with the roller.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.